High efficiency planar open magnet MRI system structured and arranged to provide orthogonal ferrorefractory effect

ABSTRACT

A planar MRI system is disclosed. The system has an open magnet configuration that produces a magnetic field having a remote region of substantial magnetic field homogeneity. Spatial encoding gradient coils and a rf coil provide MRI data for image reconstruction. The open magnet configuration has a ferromagnetic core with a substantially planar core surface layer and a longitudinal axis, and a unipolar current wire pair on a side of the ferromagnetic core adjacent the planar core surface layer. The wire pair is separated along the longitudinal axis and extends in a direction substantially perpendicular to the axis and substantially parallel to the planar core surface layer. The current wire pair provides a magnetic field having a maximum between the current wire pair along a direction perpendicular to the planar core surface layer and in the remote region of substantial magnetic field homogeneity. The planar core surface layer of the ferromagnetic core provides an orthogonal refractory effect in the form of mirror imaging current wires having the same polarity as the current wire pair that substantially increases the resulting magnetic field compared to a magnetic field generated by the current wire pair in free space.

FIELD OF THE INVENTION

[0001] This invention relates magnetic resonance imaging (MRI) systems,to magnet systems for producing a homogeneous imaging field for MRI and,particularly, to open magnet systems providing a remote region of fieldhomogeneity along with planar gradient coils for delivering gradientfields for spatial encoding in a remote target field region. Moreparticularly, this invention relates to MRI magnet systems having a pairof current loops wound about a substantially planar ferromagnetic core,the core preferably including an orthogonal end piece.

BACKGROUND OF THE INVENTION

[0002] There are known whole body MRI magnets (super-conductive,resistive iron core magnets, and permanent magnets), which produce thebackground B_(o) field used in MRI. The useable imaging volume in thesemagnets is in the region where the field is an extremum and provides aregion of substantial field homogeneity. This volume is located in theair space centrally located between field sources. Thus, typically, MRImagnets are designed to provide a homogeneous magnetic field in aninternal region within the magnet, e.g., in the air space of a largecentral bore of a solenoid or in the air gap between the magnetic polesof a C-type magnet. A patient or object to be imaged is positioned inthe homogeneous field region located in such air space. In addition tothe main or primary magnet that provides the background magnetic fieldB_(o), the MRI system typically has gradient and rf coils, which are,used respectively for spatial encoding and exciting/detecting the nucleifor imaging. These gradient field and rf coils are typically locatedexternal to the patient inside the geometry of the B_(o) magnetsurrounding the central air space.

[0003] Prior art electromagnets such as described by Watson et al andMüller et al. and other prior art iron core magnets typically have astructural design to provide a maximum magnetic field strength at alarge central air space. In addition, those types of the prior artmagnets, of the iron core electro- or permanent type, have a substantialedge fringe field effect, which makes it difficult to image beginningimmediately at the magnet edge or even proximal to the edge of themagnet due to lack of sufficient field homogeneity.

[0004] In U.S. Pat. No. 5,049,848 a magnet configuration for MRImammography is disclosed. The magnetic structure 50 has a rectangularshaped magnet with at least two parallel magnetic source 5,6 connectedby a ferromagnetic core flux path defining an air gap for imaging. Aremote shimming C-shaped magnetic source is preferably used for shimmingto decrease the front edge fringe effect of the magnetic structure 50 tocreate a relatively homogeneous field in the air gap beginning at thefront edge for effective imaging.

[0005] Solenoidal MRI magnets (superconductive, resistive) as well asiron core C and E shape electromagnets or permanent magnets are knownfor imaging of the whole body and its extremities. However, such wholebody MRI magnets are not generally well-suited for treatment of thepatient with other modalities or for minimally invasive surgicalprocedures guided by real time MRI because of the limited access of thesurgeon to the patient. This limited access results from the fieldproducing means surrounding the imaging volume. Electromagnets of the Cor E type iron core configuration have been designed to offer apartially open access to the patient, however, the access is still verylimited with typical air gaps of only 40 cm between the pole pieces of aC type magnet. U.S. Pat. No. 5,378,988 describes a MRI system, which canprovide access for a surgeon or other medical personnel, using aplurality of C-shape solenoidal magnets oriented to form an imagingvolume in a central region of the magnets.

[0006] Another type of magnet specifically designed for interventionalsurgical guidance is General Electric's Magnetic Resonance Therapydevice, which consists of two superconducting coils in a Helmholtz coiltype arrangement. (See U.S. Pat. No. 5,677,630) The air gap for thiscommercial magnet is 58 cm, which typically permits access by onesurgeon.

[0007] None of those prior art magnets or MRI systems is ideal withregard to simultaneously offering real time imaging and fully openaccess to the patient. Many surgical procedures require three or moresurgeons together with an array of supporting equipment and, thus, afully open magnet configuration for a MRI system for interventionalprocedures is desirable. In addition, such open magnet configuration isdesirable for patients that have claustrophobia.

[0008] Applications other than MRI have used magnets that produce auseful field region outside the magnet geometry. U.S. Pat. No. 4,350,955describes means for producing a cylindrically symmetric annular volumeof a homogeneous magnetic field remote from the source of the field. Twoequal field sources are arranged axially so that the axial components ofthe fields from the two sources are opposed, producing a region near andin the plane perpendicular to the axis and midway between the sourceswhere the radial component of the field goes through a maximum. A regionof relative homogeneity of the radial component of the background fieldB_(r) may be found near the maximum. The large radial field is generallydenoted as the B_(o) background field in MRI applications. See also, J.Mag. Resonance 1980, 41:400-5; J. Mag. Resonance 1980, 41:406-10; J.Mag. Resonance 1980, 41:411-21. Thus, two coils producing magneticfields having opposing direction are positioned axially in a spacedrelationship to produce a relatively homogeneous toroidal magnet fieldregion in a plane between the magnets and perpendicular to the axis ofcylindrical symmetry. This technology has been used to providespectroscopic information for oil well logging but has not been used forimaging.

[0009] U.S. Pat. No. 5,572,132 describes a magnetic resonance imaging(MRI) probe having an external background magnetic field B_(o). Theprobe has a primary magnet having a longitudinal axis and an externalsurface extending in the axial direction and a rf coil surrounding andproximal to the surface. The magnet provides a longitudinal axiallydirected field component B_(z) having an external region of substantialhomogeneity proximal to the surface. Comparing this magnet geometry tothat of U.S. Pat. No. 4,350,955, it has a background B_(o) field with acylindrically symmetrical region of homogeneity. However, this magnetdescribed in the copending application provides such a field in theaxial or z direction (i.e., longitudinal axis direction) whereas theother provides a background B_(o) field in the radial or r direction(i.e., radial direction). Preferably, the B_(o) field is provided by twomagnets spaced axially and in axial alignment in the same orientationand wherein said region of homogeneity intersects a plane that islocated between the magnets and that is perpendicular to the axis. ForMR imaging, surrounding the primary magnet are r-, z- and φ-gradientcoils to provide spatial encoding fields.

[0010] It is desirable to have new and better devices and techniques forbiomedical MRI applications such as open magnet MRI systems for imagingwhile performing surgery or other treatments on patients or for imagingpatients that have claustrophobia. It is also desirable to have portabledevices and imaging techniques that could be applied to a wide varietyof imaging uses.

[0011] U.S. Pat. No. 5,744,960 describes a planar MRI system having anopen magnet configuration comprising two pairs of planar pole piecesthat produces a magnetic field having a substantial remote region ofhomogeneity.

[0012] U.S. Pat. No. 5,914,600 describes an open solenoidal magnetconfiguration comprises a pair of primary solenoidal coils and, locatedwithin the primary coil geometry, a bias coil system, the coils emittingan additive flux in the imaging region to generate a resulting fieldwhich provides a remote region of substantial field homogeneity.

[0013] U.S. Pat. No. 6,002,255 describes an open, planar MRI systemhaving an open magnet configuration including a planar active shimmingcoil array that produces a magnetic field having a substantial remoteregion of homogeneity. The MRI system also includes spatial encodinggradient coils and a rf coil, each preferably having a planarconfiguration.

[0014] The prior art magnet configurations that provide a primarybackground magnetic field having a remote region of substantial fieldhomogeneity typically comprise a primary magnet system having spacedprimary field emission surfaces and, located between the spaced fieldemission surfaces, a bias magnet system having spaced bias fieldemission surfaces that emit an additive flux in the imaging region togenerate a resulting field which provides a remote region of substantialfield homogeneity. The spaced primary field emission surfaces typicallyare the pole pieces of a primary magnet or a solenoidal magnet facingthe target region.

[0015] However, a fundamental problem for obtaining an open magnethaving maximal accessibility for a surgeon to conduct MRI guided surgeryresults from the fact that conventional magnet systems exhibit asubstantial drop in magnet efficiency when providing an open volume ofmagnetic field that is large enough for surgery to be conducted therein.Conventional iron core C-type magnet configurations provide a targetfield volume between co-axial pole pieces. That type of magnetconfiguration, even with air gap enlargement (reducing magnetefficiency), still has limited accessibility for MRI guided surgery. MRImagnet systems in the form of one side “pancake type” magnetconfigurations (e.g. U.S. Pat. No. 5,331,282) generally have a set ofcoaxial circular coils with alternating polarity and axially shiftedpositions and provide relatively low level of remoteness and require alarge diameter magnet for adequate field strength, thus, inhibitingaccessibility to the region of field homogenity. So-called “open”solenoid superconductive magnets (e.g., U.S. Pat. No. 5,677,630) providebetter accessibility and larger field of view (FOV) but accessibilitystill is limited by axial distance between two solenoidal magnets (whichtypically is about the width of a person's shoulders). The planar openmagnet systems mentioned above (e.g., U.S. Pat. Nos. 5,378,988;5,744,960; 5,914,600 and 6,002,255) provide complete openess forexcellent accessibility but suffer still from a limitation in magnetefficiency.

[0016] Thus, there remains a need to provide a more efficient,economical, open magnet MRI system having a remote region of substantialfield homogeneity.

SUMMARY OF THE INVENTION

[0017] The present inventor has discovered the magnetic field providedby a pair of spaced parallel current wires can be refracted andmagnified substantially by a planar ferromagnetic core that isstructured and arranged to provide a substantial orthogonalferrorefraction effect, which provides the effect of generating a set ofmirror image current wires of the same polarity. It has been found thata pair of current wires spaced apart along the longitudinal axis on oneside of a flat ferromagnetic core provides a field on that side of thecore having a distinct maximum at a remote position perpendicular to thecore and between the current wires. The position of the maximum from thecore is a function of the separation between the current wires. Themagnetic coupling of the current loops (providing the current wires)with a ferromagnetic core of high permeability provides an open planarmagnet configuration having increased magnet efficiency.

[0018] The present invention provides a planar MRI system having an openmagnet configuration that produces a magnetic field having a substantialremote region of homogeneity. The MRI system includes spatial encodinggradient coils and a rf coil, each preferably having a planarconfiguration. The magnet configuration comprises a substantially planarferromagnetic core around which is wound a pair of primary coils, thecoils providing primary current wires on one side of the planar coreemitting a flux in the imaging region to generate a resulting magneticfield which provides a remote region of substantial field homogeneity.

[0019] The ferromagnetic core preferably is a plate of a material havinghigh permeability. Further, the core preferably is made by laminatinglayers of ferromagnetic material to form the substantially planarstructure of the core.

[0020] The planar ferromagnetic core provides a magnetic shield betweenthe primary current wires on one side of the core plane and the returnwires of the current loops on the opposite side of the core plane. Inaddition, the planar ferromagnetic core provides a refractive effectthat magnifies the magnetic field provided by the current wires by amirror image effect when compared to the field provided by the currentwires alone. Preferably, the distance between the primary current wiresemitting the field and the return wires of the current loops isminimized to minimize the amount of copper and the loss effects for thecoil having a particular length of current wire on the surface of theplanar core facing the target volume having a region of substantialfield homogeneity. In other words the thickness of the planar corepreferably is minimized. However, it also is preferred that the corematerial be near saturation (but not saturated) when the magnet systemis operated. Thus, the core thickness is designed preferably to preventsaturation while operating near saturation to obtain a maximum magneticinduction (i.e., B-field) in the core for the ampere turns provided.

[0021] In a preferred embodiment of the invention, opposite ends of theplanar ferromagnetic core are extended substantially perpendicular tothe plane of the core (as defined by the portion of the core lyingbetween the pair of current loops) on the side of the core on which liesthe desired remote field of substantial homogeneity. The perpendicularextension of the ends provides an additional magnification effect of theprimary current wires to the resulting field strength in the remoteregion of substantial field homogeneity. More preferably, the ends areextended at an acute angle with the plane of the core lying between thepair of current loops. The angle of the end extensions measured from theperpendicular is about 30° or less. Preferably, the angle of the endextensions measured from the perpendicular is about 20° or less. Mostpreferably, the angle of the end extensions measured from theperpendicular is in the range of about 15 to 20°.

[0022] Preferably, the end extensions also are made by laminating layersof ferromagnetic material, similar to the construction of the planarferromagnetic core.

[0023] In other preferred embodiments, more than one pair of primarycurrent wires is used to provide the desired remote region ofsubstantial field homogeneity. In addition, one or more pair of shimmingcurrent wires are used to compensate for deflections in the fieldprofile provided by the primary current wires. The shimming currentwires preferably can be located on a plane that is closer to the remoteregion of substantial field homogeneity than the plane of the primarycurrent wires. The use of a second substantially planar ferromagneticcore (which is much thinner than the primary ferromagnetic core and canbe in sections corresponding to the location of particular shimmingcurrent wires) also is preferred for the shimming current wires toobtain magnet efficiency.

[0024] The magnet configuration of the present invention can be used toprovide two back to back MRI systems because the ferromagnetic coreprovides shielding of one system from the other. Thus, a further economycan be obtained where such a back to back configuration can be utilizedto provide double patient throughput such as, for example, in massscreening applications such as mammography screening.

[0025] Preferably, the magnet configuration providing the remote regionof substantial field homogeneity is located on one side of a plane,which is parallel to the plane of the electromagnet ferromagnetic coreand separates a patient or body component to be imaged from the magnetconfiguration, thereby providing a planar open magnet configuration. Asdescribed herein, the background field has a direction substantiallyparallel to the z-axis in a rectangular coordinate system, the y-axisand z-axis define the orientation of the planar surface, and the x-axisis perpendicular to the planar surface. The z-axis may sometimes hereinbe called the longitudinal axis.

[0026] As used herein, the term “remote” means that the field region islocated external to the plane of current wires producing the magneticfield. As used herein, the terms “substantial homogeneity”, “substantialfield homogeneity” or “substantial relative field homogeneity” refer toand mean a region having magnetic field homogeneity sufficient forproducing MRI images of the desired quality.

[0027] As used herein, the term “substantially planar” means that thesurface of the core facing the desired magnetic field is a flat surface.It should be noted that the thickness of the core will depend upon thedesired magnitude of the field to be provided by the magnet, i.e., uponthe number of ampere-turns used to excite a given field.

[0028] The distance of the region of homogeneity from the planar surfacecan be controlled by varying the spacing between the primary coil pairs.The primary coil pairs are the major source for background field anddetermine the basic field strength. The shimming coil pairs providecompensation for field deflection resulting from the primary coils andcontribute to the degree of homogeneity of the background field. Thesize, geometry and spacing of the coils can influence the size of thehomogeneous region.

[0029] The MRI system also preferably has (i) a planar xyz gradient coilsystem that produces a gradient field for spatial encoding in the regionof homogeneity, i.e., the imaging volume or target region, as well as(ii) rf excitation and receiving coil (or coils).

[0030] The size of the magnet configuration and MRI system in accordwith the present invention can be varied to provide whole body imagingor portable systems for localized imaging.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1A is a cross sectional view of an isometric illustration ofa magnet configuration in accord with one embodiment of the presentinvention.

[0032]FIG. 1B is an illustration of the background magnetic field B_(o)along the x-axis provided by the magnetic configuration of FIG. 1A.

[0033]FIG. 2A is a cross sectional view of an isometric illustration ofa magnet configuration illustrating one pair of current loops coupledwith a planar ferromagnetic core in accord with the present invention.

[0034]FIG. 2B is an equivalent magnetic circuit for the magnetconfiguration of FIG. 2A.

[0035]FIG. 2C is an illustration of mirror image refractionmagnification effect provided to the field generated by a current wireby two perpendicular ferromagnetic plates positioned adjacent the lengthof the current wire.

[0036]FIG. 2D is a graph of the maximum field strength B_(max) measuredat a point along the x-axis, which is perpendicular to the core surface,for one pair of coils around a planar core (FIG. 2A) having variableincrements of thickness (t_(c)).

[0037]FIG. 3 is a cross sectional view of an isometric illustration, inpart, of a magnet configuration for a double MRI system in accord withan embodiment of the present invention.

[0038]FIG. 4 is an illustration of the longitudinal axial positioning ofcoils for x-z (2 dimensional) shimming of the remote region ofsubstantial field homogeneity.

[0039]FIG. 5A is an illustration of an experimental device used tomeasure field strength for magnet systems in accord with the presentinvention.

[0040]FIG. 5B is graph showing the relationship of maximum fieldstrength and the length of ferromagnetic end plate returns positionedorthogonal to the ferromagnetic core plate.

[0041]FIG. 5C is a graph illustrating the maximum field strength for thedevice of FIG. 5A with and without ferromagnetic end plate returns,showing the increased field strength due to the refraction effectprovided by the present invention.

[0042]FIG. 6 is a graph illustrating the affect of the angle of theferromagnetic end plate returns on the maximum field strength.

[0043]FIG. 7 is a schematic illustrating a planar remote fieldx-gradient coil in the form of an array of current loops and furtherillustrating at the center of each loop the vector magnetic dipoleassociated with the loop. Also illustrated are the field vectorsgenerated by the magnet dipoles providing an approximately constantgradient field.

[0044]FIG. 8A is a schematic plan view illustration of a planar remotefield x-gradient coil in the form of an array of current wires.

[0045]FIG. 8B is a cross sectional view of the x-gradient coil of FIG.8A along the z-axis illustrating field lines and further schematicallyillustrating the B_(z) field as a function of x at y=z=0 showing anapproximately constant gradient field with respect to x.

[0046]FIG. 8C illustrates a simple three current wire array and a simplecomputation vector diagram to aid the understanding of an X gradientfield with the current wire array.

[0047]FIG. 9A is a schematic plan view illustration of a planar remotefield y-gradient coil in the form of a current wire array.

[0048]FIG. 9B is an isometric illustration of the planar array of FIG.9A including an illustration of the remote y-gradient field provided bythe array of FIG. 9A.

[0049]FIG. 9C is an illustration of the y-gradient field profile alongthe y-axis of FIG. 9B.

[0050]FIG. 9D is an illustration showing a simple six (6) wire array fora y-gradient coil for analysis of coil capability.

[0051]FIG. 10A is a schematic plan view illustration of a planar remotefield z-gradient coil in the form of a rectangular loop array.

[0052]FIG. 10B is a schematic cross sectional view in the xz plane ofthe wires of the loop array of FIG. 10A and illustrates the remotez-gradient field provided by the z-gradient coil.

[0053]FIG. 11A is a schematic illustration of three rf coils forexcitation and detection of the nuclear magnetic resonance signal, whichprovide a remote x-directional rf field orthogonal to the z directionalbackground field.

[0054]FIG. 11B illustrates the x-directional field provided by the rfcoil of FIG. 11A which is orthogonal to the z-axis.

[0055]FIG. 12 is a front elevational view of two magnet configurationsof the present invention in a “face to face” arrangement that provides afield having a maximum approximately twice that of one configurationstanding alone.

[0056]FIG. 13A is an illustration of part of a magnet configuration inaccord with a preferred embodiment of the invention showing aconstruction to provide a layer operating near magnetic saturationadjacent to the current wires providing the primary magnetic field.

[0057]FIG. 13B is an equivalent magnetic circuit for the magnetconfiguration of FIG. 13A.

[0058]FIG. 14A is an illustration of a stepped end plate extension forproviding orthogonal refraction in accord with one embodiment of thepresent invention.

[0059]FIG. 14B is an illustration of an alternative construction for astepped end plate extension for providing orthogonal refraction inaccord with another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0060] The invention will be described with reference to the drawingswherein like reference numerals in different figures refer to the samecomponent. In accord with the present invention, a magnet configurationfor a MRI system is an open device that provides a background B_(o)magnetic field having a remote region of substantial homogeneity andoriented substantially parallel to the plane of the ferromagnetic coreportion that is between a primary pair of current wires that providesthe magnetic field.

[0061] As illustrated in FIG. 1A, one embodiment of a magnet system 10in accord with the present invention has a pair of primary current wiresdived into two portions. A first portion of primary current wires 21(provided by N₂ turns of the coil), and a second portion of primarycurrent wires 22 (provided by N₁ turns) on one side of a ferromagneticplate 25 (around which the coils are wound). Each portion of primarycurrent wires have a first group of wires having one polarity spacedapart from a second group of wires having same polarity along thelongitudinal axis (i.e., Z axis in FIG. 1A) of the ferromagnetic plate,which serves as a core for the coils. In addition, the ferromagneticplate provides high magnetic conductivity (due to high permeability ofthe material) in the core region between the primary current wires andthe return wires provided by the loops, which return wires have oppositepolarity. The ferromagnetic plate also can be considered to provide amagnetic shielding effect between the primary current wires lying on oneside of the plate and the return wires of the coils lying on theopposite side of the plate (which can be used to provide independentmagnetic field generation on opposite sides of the core plate).

[0062] Preferably, ends 26 of the ferromagnetic plate 25 are extendedorthogonal to the longitudinal axis (i.e., in the X direction in FIG.1A). The ends 26 also preferably can have a pole piece 27, which canhave a geometry to aid in shaping the magnetic field provided by themagnet system 10. However, the primary field is generated by the pairsof primary current wires.

[0063] Shimming of the magnetic field is provided by a first pair ofshimming current wire groups 31 having N_(s1) wires in each group and asecond pair of shimming current wire groups 32 having N_(s2) wires ineach group. In this embodiment, the current in the first pair ofshimming current wires 31 has opposite polarity to the second pair ofshimming current wires 32. However, the polarity can be in the samedirection in both pair shimming wires. The pairs of shimming wire groupsare wound about a ferromagnetic plate 35 and spaced along thelongitudinal axis thereof. Preferably, ferromagnetic plate 35 can besegmented to maximize the shimming field contribution from each shimmingcurrent wire and, at he same time, to minimize a shunting effect on theprimary field. Orthogonal end extensions and optionally even polepieces, similar to plate 25, may be provided in certain circumstances.Ferromagnetic plates 25 and 35 are positioned in parallel. The primaryand shimming current wires are spaced symmetrically about a centerlineof the ferromagnetic plates in the X direction (i.e., X=0),respectively. Typically, the centers of the core plates are aligned withthe X,Y plane as illustrated.

[0064] It should be noted that the shimming of the target field regioncan be either active or passive.

[0065] The magnetic field in the Z direction B_(z) along the X axis thatis provided by the magnet system 10 (FIG. 1A) is illustrated in FIG. 1B.Curve B₁ is the field provided by primary current wires 21. Curve B₂ isthe field provided by primary current wires 22. Curve B_(s1) is thefield provided by shimming current wires 31. Curve B_(s2) is the fieldprovided by shimming current wires 32. Curve B(N₁+N₂) is the sum of thefields provided by the primary current wire group pairs (B₁+B₂). CurveB(N₁+N₂+N_(s1)+N_(s2)) is the sum of the fields provided by the primaryand shimming current wire group pairs. T designates the region ofsubstantial field homogeneity of magnetic field provided by the magnetsystem.

[0066] The ferromagnetic plates preferably are made by laminatingtogether sheets of ferromagnetic material having the desired shape, e.g.the shape of a longitudinal cross section of the plate and optionallyincluding orthogonal end extensions. As in transformer construction, theuse of laminated sheets reduces induced currents (which can arise due topulsing of gradient coils in a MRI system) and provides a moretechnological capability in manufacturing the magnet.

[0067]FIG. 2A illustrates a magnet system 50 having only one pair ofcurrent wire groups 51 and a ferromagnetic plate 55 that consists of aplurality of sheets of ferromagnetic material. The thickness of theferromagnetic plate 55 is variable depending upon the number of sheetslaminated together to form the plate. A magnet system as illustrated wasconstructed to determine the effect of a ferromagnetic plate ofthickness t_(c) on the shielding of the return wires and the refractioneffect. The coils providing the current wire groups were made of 18gauge copper wire with 100 turns per coil and were spaced 8 cm apart ona core plate 30 cm long and 24 cm wide. The field was measuredperpendicular to the plate at the center of the two coils with a currentof 3 amps in each coil. The thickness of the plate t_(c) was varied inincrements of 0.3 mm, the thickness of the sheets of M6 transformersteel used for the laminations.

[0068] An equivalent lumped parameter magnetic circuit for the magnetsystem 50 of FIG. 2A is illustrated in FIG. 2B. The equivalent circuitcan be used to compute a minimum thickness of the ferromagnetic coreplate providing a maximum in the remote field. This computation is madeto differentiate a contribution generated by the current wires and thecontribution due to the mirror image effect due to refraction by theorthogonal core plate structure. Computations using the equivalentcircuit can be performed by conventional circuit analysis well known tothose skilled in the art.

[0069]FIG. 2D is a graph illustrating the maximum field strength of thecoils as a function of the number of laminated sheets. The fall off at18 laminations (i.e., sheets) is due to an artifact caused by lowsaturation because of the low current, which was kept constant for theexperiment.

[0070] The quadro mirror effect provided for a magnetic field generatedby a straight wire I₁ by mutual orthogonal ferromagnetic core plates ofinfinite length is illustrated in FIG. 2C. The presence of the mutualorthogonal ferromagnetic plates has the effect of inducing a field froma unipolar current wire pair where the field can be calculated as ifincluding a contribution from three other imaginary wire pairs (I_(2i),I_(3i), I_(4i)) located on opposite sides of the mutually orthogonalferromagnetic plates from the actual current wire I₁ as shown in thefigure. A pair of wires I₁ produce a magnetic field B_(1m) along the Xaxis. The orthogonal ferromagnetic end plates at each end of the magnetconfiguration provide an orthogonal equipotential boundary condition athigh permeability and provide an effect to magnify the magnetic fieldproduced by the actual pair of current wires I₁ alone thereby providinga total magnetic field B_(tot) that is equal to the sum of magneticfields produced as if imaginary wires I_(2i), I_(3i), I_(4i) were actualwires considered without the ferromagnetic core boundaries (this totalfield is called the “ferrorefraction” effect herein). The total magneticfield B_(tot) produced by wires I₁ and the orthogonal ferromagnetic coreplates is equal to the sum of B_(1m), B_(2im), B_(3im) and B_(4im). Inactual magnet systems where the plates cannot be infinite for practicalreasons, the magnification effect is less than theoretical.

[0071] Because of the magnetic shielding provided by the ferromagneticcore plate, the magnet structure of the present invention can be used toobtain two individual “back to back” magnet systems wherein the currentto the primary coils is used to operate both magnet systems (i.e.,providing a “double sided” magnet system). This can provide a costeffective operation mode for applications where back to back systems canbe utilized, for example, in mass screening applications such as,specifically, mammography screening. FIG. 3 illustrates an embodiment ofsuch a magnet structure. The back to back magnet system 60 has a firstpair of primary coils 61 having N_(m) turns, and a second pair of coils62 having N₁ turns coupled with a ferromagnetic core plate 65. Theprimary coils are spaced along the longitudinal axis (i.e., Z axis).

[0072] Ends 66 of the ferromagnetic core plate 65 are extendedorthogonal to the longitudinal axis (i.e., in the X direction), both tothe right and to the left of plane of the core plate 65. The ends 66also preferably have a pole pieces 67r and 67, which can have a geometryto aid in shaping the magnetic field provided by the magnet system.

[0073] The double side magnet is provided with symmetrical primarycurrent wire pairs and shimming current wire pairs for the right andleft sides. Shimming of the magnetic field produced by the primary coilsis provided on both sides of the plate 65; on the right side by a firstpair of shimming coils 71r having N_(s2r) turns and a second pair ofshimming coils 72r having N_(s1r) turns, and on the left side by a firstpair of shimming coils 71 having N_(s2) turns and a second pair ofshimming coils 72 having N_(s1) turns. The pairs of shimming coils arewound about a ferromagnetic shimming core plate 75r or 75, respectively,and spaced along the longitudinal axis thereof. Ferromagnetic shimmingcore plate 75r or 75 also can have orthogonal end extensions and polepieces, similar to plate 25 (in FIG. 1A). Ferromagnetic shimming coreplates 75r and 75 are positioned in parallel with core plate 65. Theprimary and shimming coils are spaced symmetrically about thelongitudinal centers of the ferromagnetic core plates, respectively.Typically, the longitudinal centers of the plates are aligned with theX,Y plane as illustrated. The background magnetic field B_(o) in the Zdirection along the X axis that is provided by the right side of themagnet system is illustrated. A field having the same magnitude andopposite polarity is generated on the left side.

[0074] An alternative shimming scheme for two dimensional shimming isillustrated in FIG. 4. For illustration purposes, only the positions ofthe coils along the Z axis are shown. The shimming coil system 80provides shimming of the B_(z) field in both the X and Z spatialdirections. Two pairs of shimming coils 81, 82 having different numbersof wire turns is centered about the X axis and provides a maximumshimming field along axis X₁ where in this case Z=0. Two pairs ofshimming coils 83, 84 are shifted along the Z axis in the positivedirection and centered about axis X₂. Another two pairs of shimmingcoils 85, 86 are shifted along the Z axis in the negative direction andcentered about axis X₃. Each of the three sets of coils (81,82; 83,84and 85,86) are designed to provide the same X direction shimming effect.By shifting the position of the symmetric sets of coil pairs along the Zaxis, a shimming effect also is provided in the Z direction. Thus, aregion of the magnetic field denoted by T in the X and Z directions isaffected by the three sets of shimming coils.

[0075] A device used to measure the magnetic field provided by primaryelectromagnetic current wires and a planar core is illustrated in FIG.5A. Two pair of primary coils were used. In the first pair, each coilhad 100 turns of 18 gauge copper wire and a coil width t_(N) along theZ-axis of about 2.25 cm. Each coil was positioned along the Z-axis at adistance h₁ from the X-axis, providing a separation of 2h₁ between thefirst coil pair. In the second pair, each coil had 547 turns of 18 gaugecopper wire and a coil width t_(N) along the Z-axis of 4.5 cm. Thesecoils were positioned along the Z-axis at a distance h₂ from the X-axis,providing a separation of 2h₂ between the second coil pair. The core wasmade of M6 transformer laminar steel having saturation inductanceB_(s)=1.8 T (Tesla) and the core had a total thickness of 0.8 cm (eachlamination thickness being approximately 0.3 mm), a width along theY-axis of 24 cm, and a variable length along the Z-axis of up to 30 cm.The orthogonal end extension is made of the same M6 transformer steeland extends 9 cm in the X-direction from the surface of the planar coreplate. Shimming devices can be added, if desired.

[0076] The effect of the length of the orthogonal end plate extensionswas determined by varying the length of the end plate in increments of1.0 cm from 0 to 9.0 cm. The results are presented in the graphillustrated in FIG. 5B. In the experiment, only one pair of current wiregroups each having 547 wires was used. The wire groups were positionedat h₂=8 cm. The maximum field strength was measured perpendicular to thecore plate along the X-axis.

[0077]FIG. 5C shows the maximum field strength measured experimentallyfor two pairs of coils as described above, h₂=8 cm, h₁=6 cm, and 3 ampcurrent in the coils. The device of FIG. 5A was operated with andwithout the end plate extensions (9 cm in length). The dashed linesindicate calculated values and the solid lines indicate experimentallymeasured values. As can be seen, without the end plates the deviceprovided a maximum field of about 80 gauss in a region where X=5.5 to7.0 cm (illustrated by solid squares connected by solid lines). This canbe compared with calculated field value of 40 gauss for free space,which was calculated using the Biot-Savart law methodology (illustratedby solid circles connected by dashed lines). With the end plateextension added, the device provided a maximum field strength of about150 gauss in the region where X=6.5 to 8.0 cm (illustrated by x'sconnected by solid lines). The comparison calculated values (illustratedby solid triangles connected by the dashed line) were made using aquadru mirror image methodology. It should be noted that the end plateextension causes the region of maximum field strength to shift outwardalong the X axis.

[0078] Instead of orthogonal end plates, the angle of the end platesfrom the vertical toward the X-axis was varied to determine the affectof the angle on the maximum field strength. all parameters except forthe angle of the end plates was kept constant. As illustrated in FIG. 6,the maximum field strength increases with the angle of the end plate, atleast up to an angle of 20° from the vertical.

[0079] In accord with the present invention, a MRI system comprising amagnet configuration, for example, as illustrated in FIG. 1A alsocomprises a set of xyz gradient coils to provide spatial encoding in theremote region of background B_(o) field homogeneity for imaging. Inkeeping with the planar open design of the magnet geometry andconfiguration of the present invention, planar xyz gradient coilspreferably are used. Advantageously, the remote field xyz gradient coilsare preferably planar open surface coils. Preferably, the entiregradient coil is substantially in the same plane and located in front ofthe primary and shimming current wire pairs. Each single planar gradientcoil produces a remote gradient field along the x, y or z axis in thetarget region of background field homogeneity.

[0080] As illustrated in FIG. 7, a planar remote field x-gradient coilcan be constructed using multiple current loops in a current loop arrayconfiguration. As shown, two external loop sections are configured tohave mutually opposite current polarity. Two internal current loopsections are configured also to have mutually opposite current polarityand the current polarity in the inner loop sections is also mutuallyopposite its corresponding external loop section (FIG. 7A). The arrayloops are symmetric with respect to the y and z axes.

[0081] Also illustrated on the right side of FIG. 7 is a simple fieldcomputation approximation using the magnetic moment vectors associatedwith the current loops. The basic B_(z) field components at x₁contributed by each of the four magnetic dipoles are shown. The vectorsum of the field from the two main current loops with current I₁ andturns N₁ is denoted B_(z1)and the two bias current loops with current I₂and turns N₂ is denoted B_(z2). Also illustrated on right hand side ofFIG. 7 are (i) the vector sum of the field from the current loops whichproduces a gradient field B_(z,x), which is substantially linear overthe target field region Δx₁ and which corresponds to the region in whichthe background field B_(z,o) (also shown) goes through an extremum.

[0082] An alternative x-gradient coil structure is illustrated in FIG.8A, with field lines of the two opposing fluxes illustrated in FIG. 8Bwith the resultant x-gradient field, z component, B_(z,x) which issubstantially linear over Δx₁ illustrated in the right side of FIG. 8B.This coil structure is a current wire array and will provide a remotelinear x-gradient field similar to the current loop array depicted inFIG. 7. FIG. 8C illustrates a simple three wire system for demonstrationpurposes.

[0083] The x-gradient coil structure has ampere wire distribution on aplanar surface 215 with two external unidirectional current wiresections N_(11x) and N₁₂. with a central unidirectional current sectionN_(21x) having opposite current polarity to provide a central biasfield, thereby providing a remote z-component field having substantiallyconstant gradient as illustrated in FIG. 8B.

[0084] It should be noted that the x-gradient coil geometry is similarin concept to the magnet geometry for the B_(o) background field.

[0085] As illustrated in FIG. 9A, a planar remote field y-gradient coilcan be constructed using a current wire array. As illustrated, thecurrent wire array preferably has four sections corresponding to foury,z quadrants: first having coordinates 0, −y, z; the second havingcoordinates 0, y, z; the third having coordinates 0, −y, −z; and thefourth having coordinates 0, y, −z; in other words, the quadrantboundaries are the y and z axes in the x=0 plane. Each quadrant sectionis divided in subsections along the y axis. The subsections at eachz-level (Z1, Z2, Z3 . . . ) are symmetrical about and have opposingcurrent polarity with respect to the z-axis. The subsections are alsosymmetrical about and have the same current polarity with respect to they-axis. As can be seen from FIG. 9B, the number of current wires remainsconstant over y at each level of z, however, the number of wires havingpositive and negative current polarity varies along y. The current wiresproviding the remote y-gradient field are connected by return wires(dashed in FIG. 9A) which are located far enough away from the desiredwire current array to minimize target field disturbance.

[0086]FIG. 9B illustrates the current wire array, which is located inthe x=0 plane, extends in the y direction from +y_(o) to −y_(o), andprovides a y-gradient field in a remote region. It illustrates only theoperative current wires of the array of the wiring schematic of FIG. 9A.The y-gradient field is a field directed in the same direction z as thebackground field B_(o) and which varies linearly with y. On the rightside of FIG. 9B is an illustration of the y-gradient field at the remoteplane x=x_(o). Note that the field is sinusoidal as a function of y sothat the region of linearity is localized around y=0. The region oflinearity can be specified as a fraction of y_(o) which is approximately0.5 y_(o). Thus, increasing the spatial extent of the current wire arrayin y can increase the region of linearity in y.

[0087] The gradient field depends upon y because the distribution ofcurrent wires with positive and negative currents parallel to the y axischanges with the y position. FIG. 9B illustrates the current patternchanges at y=y₁, y₂, etc. Thus, between y_(o) and y₁, the currentpattern as a function of z is constant and then changes to a differentpattern between y₁ and y₂. The field produced by each of these currentpatterns is shown as a function of x at each incremental y positionwhere the z pattern of the current wire array changes. In each case, thefield goes through an extremum at the same value of x, in this caseX_(o), however, the amplitude of the extremum changes from positive tonegative producing the gradient field.

[0088]FIG. 9C is an illustration of the gradient field profile. Becausethe field is an extremum at the remote x=x_(o) plane, it is a region ofrelative homogeneity. The region of homogeneity of the gradient fieldextends in the x direction over a distance Δx_(o).

[0089]FIG. 9D illustrates the simplest six current wire array, which isuseful as an analytical model for easy computation of the remotey-gradient field.

[0090] A planar remote field z-gradient coil can be constructed betweenthe primary current wire pairs. FIG. 10A shows the z-gradient coil as aset of current wires a₁₁, b₁ and a₁₂ at positive values of z and asymmetric set of opposite polarity current wires a₂₂, b₂ and a_(2l),located at corresponding negative values of z. The current in wiresa₁₁,a₁₂ have current in one direction with the bias wire set b₁ havingcurrent in the opposite direction. The wire sets a₂₁,a₂₂ and b₂ on theopposite side of the y-axis have currents flowing in the oppositedirections to the currents in the corresponding wire sets a₁₁,a₁₂ andb₁. A remote z-gradient field is provided in the target backgroundfield. All of the current wires in sets “a” are preferably areconnected, as illustrated, to be supplied by a single power source.Similar to the field produced by the y-gradient coil of FIG. 9A, thesetwo sets of current wires produce a field maximum, each of opposite signat the remote plane x=x_(o) at different values of z. This produces thegradient field illustrated in FIG. 10B.

[0091]FIG. 10B shows a side cross sectional view of the z-gradient coilillustrating field line orientation in the xz plane at y=0 of the wiresets illustrated in FIG. 10A with a plot of the z component of thez-gradient field B_(zg) illustrating a substantially constant gradientover a range of z.

[0092] As illustrated in FIG. 11A, a planar rf coil set useful inconnection with the present invention is illustrated, which provides aremote region of approximate rf field homogeneity wherein the majorcomponents are oriented orthogonal to the z component of the backgroundfield, i.e., oriented in the x-direction. The three coils illustratedcan be used singly or in combination with each other to produce an rffield perpendicular to the B_(o) field in different regions of thetarget volume. By varying the number of coaxial current loops in the rfcoils, which can be in different but parallel planes, the region of thex-oriented rf field homogeneity can be positioned in the region ofremote field homogeneity of the background field. FIG. 11B illustratesthe x-directional field provided by the rf coil of FIG. 11A, which isorthogonal to the z-axis. However, it will be appreciated by thoseskilled in the art that the rf field must be perpendicular to the B_(o)field (which for discussion in this specification is in the zdirection). Thus, an rf field in the y direction can also be illustratedin a similar manner and provided used for excitation and detection ofthe nuclear magnetic resonance signal.

[0093] The open magnet configuration providing a remote background fieldfor MRI can be made in any size to provide whole body capability or aportable system for more localized imaging. Shimming coils can also beused to define and position the remote region of background fieldhomogeneity.

[0094] Planar xyz gradient and rf coils preferably are positioned belowthe patient table in front of the primary and shim current wire pairsand can extend over the surface between end plate extensions. Theparticular geometry and size of the gradient and rf coils can beoptimized for each particular application, as is well known in the art.

[0095] Two individual magnet systems in accord with the presentinvention can be positioned adjacent to each other (i.e., “face toface”) a distance D apart, as illustrated in FIG. 12. In such case, themagnetic fields provided by the two individual magnet systems, onelocated on the Z₁ axis and the second on the Z₂ axis, are additive andprovide a substantially increased field strength at the with a maximumalong the X axis, the maximum being located on the Z₀ axis for theillustrated system. Shimming current wire pairs (not shown) preferablyalso are used to provide the desired field shape for the application.Gradient and rf coils (not shown) are also provided for MR imaging, asdiscussed above.

[0096] In order to obtain the refraction effect of a planarferromagnetic core, it has been discovered that it is sufficient to haveonly a thin layer at the surface of the ferromagnetic plate materialperforming near saturation to obtain orthogonal refraction effect, whilethe major portions of the core can be saturated or “oversaturated.”Therefore, a preferred ferromagnetic core structure to benefit from therefraction effect and to prevent the core from saturation at surfacelayer adjacent to the current wire pairs emitting the remote field isillustrated in FIG. 13A. The magnet system 130 comprises at least oneset of current wire pairs (one set of current wires 132 beingillustrated) and a primary ferromagnetic core plate 135 with an endplate extension 135 a (135 a′ optional for a second field, if desired).Ferromagnetic plates 136, 137 are constructed in parallel with theprimary core plate 135, but have substantially thinner plate thickness.End plate extensions 136 a, 137 a are also constructed parallel toprimary end plate extension 135 a. End plate extensions 136 a and 137 aare connected structurally and magnetically by ferromagnetic plate 138to form a closed loop. The other end (not shown) of the magnet system130 is similarly constructed in a symmetric mirror image construction.Thus, plates 136 and 137 are parts of a continuous mechanically closedmagnetic flux loop.

[0097] A bias coil constructed in three sections IN_(b1), IN_(b2) andIN_(b3) generates a magnetic flux in the closed magnetic loop. Thedirection of the magnetic flux in plate 136 and end plate extension 136a that is generated by the bias coil is opposite to the flux generatedby the primary current wire pairs 132. One or more bias coil sectionsare used to provide a bias flux in the magnetic loop preferably tomaintain the total magnetic flux in plates 136 and end plate 136 a at ornear saturation of the B field. Thus, plate 136 and end plate 136 aprovide substantially orthogonal refraction of the primary current wirepairs and provide independent magnetic field generation for the currentwire pairs on the right of the core from the magnetic field generationof the current wire pairs on the left of the core plate 136. The biascoils provide a demagnetization flux to compensate for the saturationinduction of the primary field coils and maintain the surface layer nearsaturation.

[0098] Core plates 135, 135 a, 137 and 137 a typically are made of M6transformer steel having a saturation inductance with B_(s)=1.8 T.Preferably, core plates 136 and 136 a are made of steel having a highersaturation inductance with B_(s)=2.2 to 2.5 T, as well as a higherpermeability prior to saturation. An equivalent magnetic circuit for themagnet system 130 is illustrated in FIG. 13B to provide an analyticaltool for design optimization. Shimming, gradient and rf coils, asdiscussed above, are provided for MR imaging. Optionally, core plates136′, 136 a′, 137′ and 137 a′ are provided along with suitable biascoils, if a “back to back” or double magnetic field system is desired.

[0099] It is known that certain magnetic properties of materials can berepresented by a B vs H curve where B is the magnetic induction and H isthe magnetic intensity. The ferromagnetic core material preferablyoperates close to saturation for the material used. Typically, close tosaturation means that the system is designed, structured and arranged sothat the ferromagnetic core layer adjacent to the primary current wirepairs operates with the ferromagnetic material having a magneticintensity H within about 20% of H_(sat), more preferably within 10% ofH_(sat), most preferably within 5% of H_(sat).

[0100] Alternative stepped end plate extension structures for providingan orthogonal quadru ferrorefraction effect are illustrated in FIGS.14A, 14B. In FIG. 14A, a primary ferromagnetic core plate 140 with astepped end plate extension 140 a ( only one end of magnet system beingillustrated) accommodates primary current wire pairs that are dividedinto four sections 142 a. 142 b, 142 c and 142 d. In this embodiment,the dimensions of the steps are shown to be the same for each step.Thus, some of the mirror images of the coils are overlapping, providingtarget field magnification. In FIG. 14B, a primary ferromagnetic coreplate 150 with a stepped end plate extension 150 a accommodates primarycurrent wire pairs 152 a. 152 b, 152 c and 152 d that are located insteps having non-equal dimensions. Thus, in this embodiment, thedimensions of the steps vary for each step to obtain the most efficientoverlap of the mirror images to generate the desired remote field andobtain maximum target field magnification. In FIGS. 14A and 14B, onlyone end of the magnet system is illustrated (compare FIG. 3). Also, adouble magnet system can be constructed using the same primary coils byproviding a mirror image structure on each side of the ferromagneticcore plate. Shimming coils can also be used as discussed above.

[0101] For the general case, the ladder step end plate extensionstructures are designed with different dimensions for each step to shapethe remote field and provide desired homogeneity for particularapplications. Larger numbers of current wire pairs typically will bepositioned at a larger distance from the X-axis (i.e., largerseparations along the longitudinal axis). The design also will beoptimized for the particular geometric constraints.

[0102] The invention has been described in detail with reference topreferred embodiments thereof. However, it will be appreciated that,upon consideration of the present specification and drawings, thoseskilled in the art may make modifications and improvements within thespirit and scope of this invention as defined by the claims. Forexample, other magnet configurations can be use for providing theprimary field or for shimming. Indeed, the current wire pairs generatingthe primary field or the shimming field can be divided into any numberof groups having different numbers of wires and different spacings alongthe longitudinal axis.

What is claimed is:
 1. A planar MRI system having an open magnetconfiguration that produces a magnetic field having a remote region ofsubstantial magnetic field homogeneity, spatial encoding gradient coilsand a rf coil, the open magnet configuration comprising: a ferromagneticcore having a substantially planar core surface layer and a longitudinalaxis; and a unipolar current wire pair on a side of the ferromagneticcore adjacent said planar core surface layer, the wire pair beingseparated along said longitudinal axis and extending in a directionsubstantially perpendicular to the axis and substantially parallel tothe planar core surface layer, the current wire pair providing amagnetic field having a maximum between the current wire pair along adirection perpendicular to said planar core surface layer and in saidremote region of substantial magnetic field homogeneity, the planar coresurface layer of the ferromagnetic core providing an orthogonalrefractory effect that substantially increases the resulting magneticfield compared to a magnetic field generated by the current wire pair infree space.
 2. The MRI system of claim 1, wherein the ferromagnetic corecomprises a plurality of layers of ferromagnetic material including saidplanar core surface layer, which is adjacent said current wire pair,said magnet configuration being constructed and adapted such that thecore surface layer operates near a magnetic saturation value of theferromagnetic material forming the core surface layer.
 3. The MRI systemof claim 2, wherein the core surface layer comprises a ferromagneticmaterial having a magnetic property including a H_(sat) value and thecore surface layer operates within about 20% of the H_(sat) value. 4.The MRI system of claim 2, wherein the core surface layer comprises aferromagnetic material having a magnetic property including a H_(sat)value and the core surface layer operates within about 10% of theH_(sat) value.
 5. The MRI system of claim 2, wherein the core surfacelayer comprises a ferromagnetic material having a magnetic propertyincluding a H_(sat) value and the core surface layer operates withinabout 5% of the H_(sat) value.
 6. The MRI system of claim 2, wherein thecore surface layer comprises a first ferromagnetic material and at leastone of the layers comprises a second ferromagnetic material, wherein thefirst ferromagnetic material has a higher saturation induction andpermeability than the second ferromagnetic material.
 7. The MRI systemof claim 1, further comprising a ferromagnetic end extension extendingfrom the plane of planar core surface layer, the end extension beingpositioned adjacent to a wire of and longitudinally exterior of thecurrent wire pair.
 8. The MRI system of claim 7, wherein theferromagnetic end extension comprises a plurality of layers offerromagnetic material including an end extension surface layer that isadjacent a wire of said current wire pair, said magnet configurationbeing constructed and adapted such that the end extension surface layeroperates near a magnetic saturation value of the ferromagnetic materialforming the end plate surface layer.
 9. The MRI system of claim 8,wherein the end extension surface layer comprises a ferromagneticmaterial having a magnetic property including a H_(sat) value and theend extension surface layer operates within about 20% of the H_(sat)value.
 10. The MRI system of claim 8, wherein the end extension surfacelayer comprises a ferromagnetic material having a magnetic propertyincluding a H_(sat) value and the end extension surface layer operateswithin about 10% of the H_(sat) value.
 11. The MRI system of claim 8,wherein the end extension surface layer comprises a ferromagneticmaterial having a magnetic property including a H_(sat) value and theend extension surface layer operates within about 5% of the H_(sat)value.
 12. The MRI system of claim 7, wherein the end extension surfacelayer comprises a first ferromagnetic material and at least one of thelayers comprises a second ferromagnetic material, wherein the firstferromagnetic material has a higher saturation and permeability than thesecond ferromagnetic material.
 13. The MRI system of claim 7, whereinthe ferromagnetic end plate extends perpendicular to the ferromagneticcore.
 14. The MRI system of claim 7, wherein the ferromagnetic end plateextends at an acute angle from the perpendicular to the planar coresurface layer and in a direction toward the current wire pair.
 15. TheMRI system of claim 14, wherein said angle is between 0 and about 20°.16. The MRI system of claim 1, further comprising a shimming currentwire pair located on a side of the ferromagnetic core closest to saidplanar core surface layer.
 17. The MRI system of claim 16, furthercomprising a ferromagnetic shimming core having a planar shimming coresurface layer adjacent to said shimming current wire pair.
 18. An MRIsystem having an open back to back magnet configuration that producestwo independent magnetic fields, each having a remote region ofsubstantial magnetic field homogeneity, the system comprising spatialencoding gradient coils and a rf coil for each remote region, the openmagnet configuration comprising: a ferromagnetic core having alongitudinal axis, a first and a second side, each side havingsubstantially planar core surface layer; and a unipolar current wirepair on each side of the ferromagnetic core adjacent said planar coresurface layer, the wire pair being separated along said longitudinalaxis and extending in a direction substantially perpendicular to theaxis and substantially parallel to the planar core surface layer,wherein said unipolar current wire pair on each side of theferromagnetic core are provided by a pair of current loops wound aroundthe ferromagnetic core; each current wire pair providing a magneticfield having a maximum between the current wire pair along a directionperpendicular to said planar core surface layer and in said remoteregion of substantial magnetic field homogeneity, the planar coresurface layer of the ferromagnetic core adjacent each current wire pairproviding an orthogonal refractory effect that substantially increasesthe resulting magnetic field compared to a magnetic field generated bythe current wire pair in free space.
 19. A MRI system having two face toface open magnet configurations that each produce a magnetic fieldhaving a remote region of substantial magnetic field homogeneity,spatial encoding gradient coils and a rf coil, wherein a first and asecond open magnet configuration each comprise: a ferromagnetic corehaving a substantially planar core surface layer and a longitudinalaxis; and a unipolar current wire pair on a side of the ferromagneticcore adjacent said planar core surface layer, the wire pair beingseparated along said longitudinal axis and extending in a directionsubstantially perpendicular to the axis and substantially parallel tothe planar core surface layer, the current wire pair providing amagnetic field having a maximum between the current wire pair along adirection perpendicular to said planar core surface layer and in saidremote region of substantial magnetic field homogeneity, the planar coresurface layer of the ferromagnetic core providing an orthogonalrefractory effect that substantially increases the resulting magneticfield compared to a magnetic field generated by the current wire pair infree space, wherein the remote region of substantial magnetic fieldhomogeneity provided by a first open magnet configuration overlaps theremote region of substantial magnetic field homogeneity provided by asecond open magnet configuration, thereby providing a total magneticfield equal to the sum of the remote regions provided by the first andthe second open magnet configurations.